Graphene is an extremely thin sheet which has a thickness of one carbon atom, and has physical properties and characteristics which exceed those of the materials in the related art, such as high intensity, high conductivity, transparency, or a high thermal conductivity. In particular, the graphene has high transparency or high conductivity, and it is highly required that the graphene is used in a transparent electrode of a solar cell, a touch panel, or a capacitor.
In particular, capacitor performance is determined by an energy density and a charge and discharge rate which correspond to an energy storage capacity, and a power density which corresponds to an instantaneous output power. The energy density is proportional to a surface area of a capacitor electrode, and the power density depends on the conductivity. As illustrated in Table 1, the graphene has larger specific surface area and extremely excellent conductivity compared to activated carbon powder or a carbon nanotube. Accordingly, when the characteristics of the graphene is sufficiently used, it is possible to develop an electric double layer capacitor (super capacitor) having high performance which has not existed until now. For this reason, the graphene has been getting attention.
TABLE 1Specific surface areaConductivityElectrode material(m2/g)(S/cm)Graphene2630106Activated carbon powder 300-2200300Carbon nanotube120-500104-105
Methods to preparing graphene are largely divided into a chemical vapor deposition (CVD) method and an exfoliation method.
A single-layered graphene sheet can be prepared by the CVD method. However, since the CVD method is a high-cost preparing method, the CVD method cannot be used for an industrial application.
The exfoliation method is a method for exfoliating the graphene from inexpensive graphite. Since mass production is possible at a low cost by the exfoliation method, the exfoliation method is appropriate for the industrial application. For this reason, in research and development for the application, the graphene prepared by exfoliation method is used.
As the exfoliation method, the following three types of methods are known.
The first method is a chemical exfoliation method, the second method is an organic solvent exfoliation method (NPL 1), and the third method is an electrolytic exfoliation method (PTL 1, NPL 2 to 6).
The chemical exfoliation method which is the first method is a general method for mass production of the graphene, and is a method for immersing graphite in which the graphene is multi-layered in a strong acid, making the graphite oxidized and expanded, exfoliating the multi-layered graphene as graphene oxide, and making the graphene by reducing the graphene oxide.
FIG. 1 is a view illustrating the chemical exfoliation method.
First, by oxidizing graphite powder in a concentrated sulfuric acid by using sodium nitrate and potassium permanganate, the graphene oxide is exfoliated. The surface of the exfoliated graphene oxide is modified by a carbonyl group, a carboxyl group, and a hydroxyl group.
Then, by performing partial reduction by using hydrazine, partially reduced graphene is made.
When the graphene oxide is partially reduced, the carbonyl group is removed, but the carboxyl group and the hydroxyl group remain. For this reason, conductivity or transparency of the graphene prepared by the reduction is not high. In addition, it is necessary to take a long time for an oxidation-reduction process. Furthermore, there are an environmental problem and a safety problem due to the strong acid or the hydrazine.
The organic solvent exfoliation method which is the second method is a method for immersing the graphite in an organic solvent of which surface tension is the same level as that of the graphene, and exfoliating the graphene by interaction with the organic solvent.
As the organic solvent which is used in the organic solvent exfoliation method and directly exfoliates the graphene from the graphite by immersing in liquid, several types of solvents may be used. Among these, the most general organic solvent is N-methylpyrrolidone (referred to as NMP).
First, after dispersing the graphite powder in NMP liquid, heating is performed for three days at 200° C. in an autoclave which is covered with Teflon (registered trademark). After this, by performing ultrasonic centrifugal separation processing, exfoliated graphene is obtained. The procedure is easier compared to that of the chemical exfoliation method, but there is a problem that productivity is low, and there are an environmental problem and a safety problem.
The electrolytic exfoliation method which is the third method is a method for using the graphite as an electrode, electrolyzing the graphite in an electrolytic solution, intercalating ions of the electrolytic solution between graphite layers, and exfoliating the graphene from the graphite electrode.
FIG. 2 is a schematic view of an apparatus which uses the electrolytic exfoliation method in the related art. The apparatus which uses the electrolytic exfoliation method is an extremely simple apparatus.
By using the apparatus, by using the graphite electrode as an anode and a platinum electrode as a cathode, a voltage which is approximately 10 V is applied between the two electrodes from a power source via wirings. Accordingly, the graphene is exfoliated from a surface of the graphite electrode, and as illustrated in FIG. 2, the graphene is aggregated on the electrode surface.
After this, these graphenes are detached from the surface of the graphite electrode, are released in the electrolytic solution, float in the electrolytic solution, and are precipitated.
FIG. 3 is a view illustrating a state of floating and precipitation in the electrolytic solution of the graphene which is electrolyzed and exfoliated.
As illustrated in FIG. 3, the graphene which is detached from the graphite electrode floats in the electrolyte liquid, and is further precipitated on the bottom.
Since modification of the carboxyl group or the like is small, the characteristics of the exfoliated graphene, such as conductivity or transparency, are excellent, and a defect or damage is also small.
The electrolytic exfoliation method is a method which can continuously prepare graphene having high performance in a short period of time, and can efficiently perform mass production at a low cost. Specifically, the electrolytic exfoliation method can prepare graphene in a shorter period of time which is equal to or less than one tenth of the time of the chemical exfoliation method. Since both the electrolytic exfoliation method and the chemical exfoliation method use the graphite as a raw material, and the apparatus thereof is inexpensive and simple, if the preparing procedure is managed mainly by preparing time, it is possible to calculate that the cost of the graphene which is prepared by the electrolytic exfoliation method is one tenth of the cost of the graphene which is prepared by the chemical exfoliation method.
For this reason, the electrolytic exfoliation method is the most desired method among the above-described three methods.
Meanwhile, in applying and employing the graphene, a thick graphene piece in which 10 or more graphenes are overlapped is not preferable, and a thin graphene piece in which less than 10 graphenes are overlapped is preferable. A thin graphene piece which is exfoliated to one single layer is more preferable. Accordingly, a method which can prepare a large quantity of thin graphene pieces in a short period of time is desired.
However, in the electrolytic exfoliation method in the related art, since the exfoliating of graphene is performed in a short period of time, intercalating of ions of the electrolytic solution is not sufficient, exfoliating of the graphene is not sufficient, and many graphenes are overlapped.
In other words, before the ions of the electrolytic solution between each layer of the graphene which constitutes the graphite electrode are sufficiently intercalated, the exfoliating is started. For this reason, the thick graphene piece in which 10 or more graphenes are overlapped is detached from the electrode, and now, a voltage is not applied to the thick graphene piece which is detached from the electrode any more. Accordingly, while a state where 10 or more graphenes are overlapped is maintained, the exfoliated thick graphene piece is released into the electrolytic solution, floats, and is precipitated. For this reason, finally, there is a problem that the thick graphene piece in which 10 or more graphenes are overlapped is collected. Furthermore, in a state where the thick graphene piece floats or is precipitated, a case where the graphenes are adhered again and recombined to each other, and return to the original graphite is also occurred.
In addition, in the application of the capacitor, in order to prevent the graphenes from being recombined and regenerated as the original graphite, a method which makes a spacer made of nanoparticles interpose between the graphenes is suggested (PTL 2 and 3).
In addition, NPL 3 to 5, and 7 to 10 are as follows.
NPL 3 relates to synthesis of graphene by electrochemical exfoliation.
NPL 4 relates to an electrode which is covered with a graphene nanosheet by the electrochemical exfoliation, and discloses an anode exfoliation method which uses a PSS solution.
NPL 5 relates to preparing of a graphene film by the electrochemical exfoliation, and discloses an anode exfoliation method which uses a sulfuric acid.
NPL 7 relates to a method for synthesizing the graphene by controlling a thickness by using an electrochemical process, and discloses an anode exfoliation method.
NPL 8 relates to preparing of the graphene by electrochemical intercalation to the graphite and ultrasound assist expansion, and discloses intercalation of HClO4 and an ultrasonic irradiation method in an NMP solution. In FIG. 4, a thick graphene piece having 10 layers is disclosed.
NPL 9 relates to synthesis of graphene flake by electrochemical expansion of the graphite in propylene carbonate, and discloses a method for performing negative charging by Li+/PC.
NPL 10 relates to graphitic platelets which are prepared by electrochemical exfoliation of the graphite, and discloses a method for discharging by LiPF6/TMP. In FIG. 3, a substantially thick graphene piece is disclosed.